text: By-products of coal combustion are compounds that are released into the atmosphere as a result of burning coal. Coal includes contaminants such as sulfur compounds and non-combustible minerals. When coal is burned, the minerals become ash (i.e. particulate matter or PM), and the sulfur forms sulfur dioxide. Since air is mostly nitrogen, the combustion of coal often leads to the production of nitrogen oxides. Sulfur dioxide and nitrogen oxides are the primary causes of acid rain. For many years—before greenhouse gasses were widely understood to be a threat—it was thought that these by-products were the only drawback to using coal. These by-products are still a problem, but they have been greatly diminished in most advanced countries due to clean air regulations. It is possible to remove most of the sulfur dioxide, nitrogen oxides, and particulate matter (PM) emissions from the coal-burning process. For example, various techniques are used in a coal preparation plant to reduce the amount of non-combustible matter (i.e. ash) in the coal before burning. During combustion, fluidized bed combustion is used to reduce sulfur dioxide emissions. After burning, particulate matter (i.e. ash and dust) can be reduced using an electrostatic precipitator, and sulfur dioxide emissions can be further reduced with flue-gas desulfurization. Trace amounts of radionuclides are more difficult to remove.
  Coal-fired power plants are the largest aggregate source of the toxic heavy metal mercury: 50 tons per year come from coal power plants out of 150 tons emitted nationally in the US and 5000 tons globally. However, according to the United States Geological Survey, the trace amounts of mercury in coal by-products do not pose a threat to public health. A study in 2013 found that mercury found in the fish in the Pacific Ocean could possibly be linked to coal-fired plants in Asia.